Datasheet

LO1

PGND

VCC

BST1

HO1

HS1

VIN

LM5045

Hiccup Mode off-time

Soft-Start

Restart delay

Count to Eight

LM5045

SNVS699G –FEBRUARY 2011–REVISED MARCH 2013

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Figure 30. Hiccup Mode Delay and Soft-Start Timing Diagram

Augmenting the Gate Drive Strength

The LM5045 includes powerful 2A integrated gate drivers. However, in certain high power applications (>500W),

it might be necessary to augment the strength of the internal gate driver to achieve higher efficiency and better

thermal performance. In high power applications, typically, the I

xR loss in the primary MOSFETs is significantly

higher than the switching loss. In order to minimize the I

xR loss, either the primary MOSFETs are paralleled or

MOSFETs with low R

DS (on)

are employed. Both these scenarios increase the total gate charge to be driven by

the controller IC. An increase in the gate charge increases the FET transition time and hence increases the

switching losses. Therefore, to keep the total losses within a manageable limit the transition time needs to be

reduced.

Generally, during the miller capacitance charge/discharge the total available driver current is lower during the

turn-off process than during the turn-on process and often it is enough to speed-up the turn-off time to achieve

the efficiency and thermal goals. This can be achieved simply by employing a PNP device, as shown in

Figure 31, from gate to source of the power FET. During the turn-on process, when the LO1 goes high, the

current is sourced through the diode D1 and the BJT Q1 provides the path for the turn-off current. Q1 should be

located as close to the power FET as possible so that the turn-off current has the shortest possible path to the

ground and does not have to pass through the controller.

Figure 31. Circuit to Speed-up the Turn-off Process

Product Folder Links: LM5045